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| United States Patent |
6,148,019
|
|
Fishman
, et al.
|
November 14, 2000
|
Modular high power induction heating and melting system
Abstract
A system for melting metal and holding molten metal, comprises a rectifier
unit receiving AC electric power and outputting DC electric power, a
plurality of inverter units, each receiving the DC electric power output
by the rectifier unit and outputting AC electric power; and a plurality of
induction furnaces each receiving the electric power output by a
respective inverter unit. Each inverter unit comprises a plurality of
inverter modules connected in parallel, each module independently being
connectable to and disconnectable from the rectifier unit and the furnace.
The rectifier unit comprises a plurality of rectifier modules connected in
parallel, each module independently being connectable to and
disconnectable from the AC supply and the inverter units. The total power
output of the rectifier unit is more than the sum of the powers required
by all of the furnaces when they are holding a charge of molten metal, but
less than the sum of the maximum powers required by each furnace when
melting a charge of metal. If the total power demand is greater than the
rectifier can supply, the power supply is reduced to those furnaces that
are receiving more than the holding power.
| Inventors:
|
Fishman; Oleg S. (Maple Glen, PA);
Mortimer; John H. (Medford, NJ)
|
| Assignee:
|
Inductotherm Corp. (Rancocas, NJ)
|
| Appl. No.:
|
418884 |
| Filed:
|
October 15, 1999 |
| Current U.S. Class: |
373/147; 219/661; 373/149 |
| Intern'l Class: |
H05B 006/06 |
| Field of Search: |
373/147,148,149,150,138
219/663,662,665,666,661
|
References Cited
U.S. Patent Documents
| 2451518 | Oct., 1948 | Strickland, Jr. | 373/147.
|
| 4403327 | Sep., 1983 | Granstrom et al. | 373/147.
|
| 5272719 | Dec., 1993 | Cartlidge et al. | 373/138.
|
| 5508497 | Apr., 1996 | Fabianowski et al. | 219/663.
|
| 5666377 | Sep., 1997 | Havas et al. | 373/147.
|
Other References
John H. Mortimer, P.E., Tomorrow's Induction Melt Shop Technologies Today,
Foundry Management & Technology, Mar. 1999, pp. 14-16, 19, and 20.
John H. Mortimer, P.E., Tomorrow's Induction Melt Shop Technologies Today,
Foundry Management & Technology, May 1999, pp. 41, 44, 46, 48 and 50.
|
Primary Examiner: Hoang; Tu Ba
Attorney, Agent or Firm: Seidel, Gonda, Lavorgna & Monaco, PC
Parent Case Text
This application claims the benefit of U.S. Provisional Ser. No. 60/133,308
filed May 10, 1999.
Claims
What is claimed is:
1. A system for melting metal and holding molten metal at a selected
temperature, comprising:
an induction furnace having a maximum power consumption;
a rectifier unit arranged to receive an AC electric supply power and to
output a DC electric power; and
a plurality of inverter modules selectively connected in parallel to form
an inverter unit, each module of said plurality of inverter modules having
a selectively connectable input to a DC common connection for receiving
said DC electric power from said rectifier unit and a selectively
connectable output to an AC common connection for delivering an AC
electric power to said induction furnace, each module of said plurality of
inverter modules having an electrical capacity less than said maximum
power consumption of said induction furnace;
wherein said AC electric power from said plurality of inverter modules is
used to selectively melt metal or hold molten metal at a selected
temperature.
2. A system according to claim 1 wherein each module of said plurality of
inverter modules is capable of delivering a substantially equal amount of
power to said induction furnace wherein the sum of said substantially
equal amount of power over all of said plurality of inverter modules does
not exceed said maximum power consumption of the induction furnace.
3. A system according to claim 1 further comprising a plurality of module
controllers, each controller of said plurality of module controllers
exclusively dedicated to an associated module of said plurality of
inverter modules, each controller of said plurality of module controllers
further comprising:
means for communication with all other of said plurality of module
controllers for cooperative operation among said plurality of module
controllers; and
means to automatically and selectively remove from service said associated
module of said plurality of inverter modules.
4. A system according to claim 3 wherein each controller of said plurality
of module controllers farther comprises means to automatically and
sequentially shut down said plurality of inverter modules when at least
one of said plurality of inverter modules is removed from service.
5. A system according to claim 3 wherein each controller of said plurality
of module controllers further comprises means to automatically supply
reduced power from said inverter unit when at least one of said plurality
of inverter modules is removed from service.
6. A system for melting metal and holding molten metal at a selected
temperature, comprising:
a plurality of induction furnaces, each furnace of said plurality of
induction furnaces having a maximum power consumption, and each furnace of
said plurality of induction furnaces arranged to be supplied with AC
electric power from a respective one of a plurality of AC connections;
a rectifier unit arranged to output a DC electric power to a DC common
connection; and
a plurality of inverter units equal in number to said plurality of
induction furnaces, each inverter unit of said plurality of inverter units
arranged to deliver AC electric power to a respective one of said
plurality of AC connections for delivering the AC electric power to one of
said plurality of induction furnaces;
each inverter unit of said plurality of inverter units further comprising a
plurality of inverter modules selectively connected in parallel, each
inverter module having a selectively connectable input to said DC common
connection for receiving said DC electric power and each inverter module
having a selectively connectable output to the one of said plurality of AC
connections that is respective to the inverter unit comprising such
inverter module;
wherein said AC electric power from each inverter unit of said plurality of
inverter units is used to selectively melt metal or hold molten metal at a
selected temperature in the respective induction furnace.
7. A system according to claim 6 wherein said DC electric power is less
than the sum of said maximum power consumption of each furnace for all of
said plurality of induction furnaces.
8. A system according to claim 7, wherein each furnace of said plurality of
induction furnaces has a furnace maximum holding power and said DC
electric power exceeds the sum of said furnace maximum holding power of
each furnace for all of said plurality of induction furnaces.
9. A system according to claim 8, further comprising a controller
operatively connected to each inverter unit of said plurality of inverter
units, said controller further comprising:
means for inputting a power set point, P.sub.i, for each furnace of said
plurality of induction furnaces; and
means for reducing the power set point, P.sub.i, for each furnace of said
plurality of induction furnaces for which said power set point, P.sub.i,
for each furnace exceeds said furnace maximum holding power, if the sum of
said power set point, P.sub.i, for each furnace over all of said plurality
of induction furnaces is greater than said DC power.
10. A system according to claim 9 wherein said controller further
comprises:
means for determining a total available power to the system from the
product of a percentage of demand limitation, D%, and a total maximum
power, P.sub.total, available to the system;
means for determining a total power required for holding,
.SIGMA.P.sub.hold, by summing the power set point, P.sub.i, for each
furnace of said plurality of induction furnaces having said power set
point, P.sub.i, less than or equal to said furnace maximum holding power;
means for determining a total power required for melting,
.SIGMA.P.sub.melt, by summing the power set point, P.sub.i, for each
furnace of said plurality of induction furnaces having said power set
point, P.sub.i, greater than said furnace maximum holding power;
means for determining a total power available for melting, P.sub.available,
from the equation:
P.sub.available =[D%.circle-solid.P.sub.total ]-.SIGMA.P.sub.hold ;
means for determining a power availability coefficient, k.sub.available,
from the equation:
k.sub.available =[P.sub.available ]/[.SIGMA.P.sub.melt ]; and
means for adjusting said power set point, P.sub.i, for each furnace of said
plurality of induction furnaces for which said power set point, P.sub.i,
is greater than said furnace maximum holding power to a value
substantially equal to the product of said power availabilitycoefficient,
k.sub.available, and said power set point, P.sub.i, if said power
availability coefficient, k.sub.available, is less than unity.
11. A system according to claim 6, wherein said rectifier unit further
comprises a plurality of rectifier modules selectively connected together
to a selectively connectable input for receiving said AC electric supply
power and a selectively connectable output to said DC common connection
for supplying said DC electric power to said plurality of inverter units.
12. A system for melting metal and holding molten metal at a selected
temperature, comprising:
a plurality of induction furnaces, each furnace of said plurality of
induction furnaces having a maximum power consumption;
a plurality of rectifier modules selectively connected together to form a
rectifier unit, each module of said plurality of rectifier modules having
a selectively connectable input to a multi-phase supply of AC power and a
selectively connectable output to a DC common connection to supply a DC
electric power, each module of said plurality of rectifier modules further
comprising:
means for transforming said multi-phase supply of AC power, said means for
transforming selectively phase shifting said multi-phase supply of AC
power to provide a selective phase-shifted output; and
means for rectifying said selective phase-shifted output; and
a plurality of inverter units, equal in number to said plurality of
induction furnaces, each inverter unit of said plurality of inverter units
dedicatedly delivering an AC electric power to a respective furnace of
said plurality of induction furnaces, and each inverter unit of said
plurality of inverter units further comprising a plurality of inverter
modules selectively connected in parallel, each module of said plurality
of inverter modules having a selectively connectable input to a DC common
connection for receiving said DC electric power and a selectively
connectable output to an AC common connection for delivering an AC
electric power to the respective furnace of said plurality of induction
furnaces;
wherein said AC electric power from each inverter unit of said plurality of
inverter units is used to selectively melt metal or hold molten metal at a
selected temperature in the respective dedicated furnace of said plurality
of induction furnaces.
13. A system according to claim 12 further comprising a plurality of
rectifier module controllers, each controller of said plurality of
rectifier module controllers exclusively associated with an associated
module of said plurality of rectifier modules, each controller of said
plurality of rectifier module controllers further comprising:
means for communication with all other of said plurality of rectifier
module controllers for cooperative operation among said plurality of
rectifier module controllers; and
means to automatically and selectively remove from service said associated
module of said plurality of rectifier modules.
14. A system according to claim 13 wherein each controller of said
plurality of module controllers further comprises means to automatically
and sequentially shut down said plurality of rectifier modules when at
least one of said plurality of rectifier modules is removed from service.
15. A system according to claim 13 wherein each controller of said
plurality of module controllers further comprises means to automatically
supply reduced power from said plurality of rectifier modules when at
least one of said plurality of rectifier modules is removed from service.
16. A system for melting metal and holding molten metal at a selected
temperature, comprising:
a plurality of induction furnaces, each furnace of said plurality of
induction furnaces having a furnace maximum holding power to hold a
substantially full charge of metal in the molten state at a pre-selected
temperature and a maximum power consumption;
a rectifier unit outputting a magnitude of DC electric power wherein said
magnitude of DC electric power is less than the sum of the maximum power
consumption for each furnace for all of said plurality of induction
furnaces, and said magnitude of DC electric power exceeds the sum of said
furnace maximum holding power for each furnace for all of said plurality
of induction furnaces;
a plurality of inverter units, equal in number to said plurality of
induction furnaces, each inverter unit of said plurality of inverter units
dedicatedly delivering an AC electric power to one of said plurality of
induction furnaces, and each inverter unit of said plurality of inverter
units further comprising a plurality of inverter modules selectively
connected in parallel, each module of said plurality of inverter modules
having a selectively connectable input to a DC common connection for
receiving said DC electric power and a selectively connectable output to
an AC common connection for delivering an AC electric power to one of said
plurality of induction furnaces; and
a control means operatively connected to each inverter unit of said
plurality of inverter units, said control means further comprising:
means for inputting a power set point, P.sub.i, for each furnace of said
plurality of induction furnaces;
means for reducing the power set point, P.sub.i, for each furnace of said
plurality of induction furnaces for which said power set point, P.sub.i,
exceeds said furnace maximum holding power, if the sum of said power set
point, P.sub.i, for each furnace for all of said plurality of induction
furnaces is greater said than said magnitude of DC power;
means for determining a total available power to the system from the
product of a percentage of demand limitation, D%, and a total maximum
power, P.sub.total, available to the system;
means for determining a total power required for holding,
.SIGMA.P.sub.hold, by summing the power set point, P.sub.i, for each
furnace of said plurality of induction furnaces having said power set
point, P.sub.i, less than or equal to said furnace maximum holding power;
means for determining a total power required for melting,
.SIGMA.P.sub.melt, by summing the power set point, P.sub.i, for each
furnace of said plurality of induction furnaces having said power set
point, P.sub.i, greater than said furnace maximum holding power;
means for determining a total power available for melting, P.sub.available,
from the equation:
P.sub.available =[D%.circle-solid.P.sub.total ]-.SIGMA.P.sub.hold ;
means for determining a power availability coefficient, k.sub.available,
from the equation:
k.sub.available =[P.sub.available ]/[.SIGMA.P.sub.melt ]; and
means for adjusting said power set point, P.sub.i, for each furnace of said
plurality of induction furnaces for which said power set point, P.sub.i,
is greater than said furnace maximum holding power to a value
substantially equal to the product of said power availability coefficient,
k.sub.available, and said power set point, P.sub.i, if said power
availability coefficient, k.sub.available, is less than unity.
17. A system according to claim 16 wherein said rectifier unit further
comprises a plurality of rectifier modules selectively connected together
to a selectively connectable input for receiving said AC electric supply
power and a selectively connectable output to a common DC connection for
supplying said magnitude of DC electric power to said plurality of
inverter modules.
18. A method for selectively melting metal and holding molten metal at a
selected temperature in a plurality of induction furnaces, each furnace of
said plurality of induction furnaces having a furnace maximum holding
power to hold a substantially fall charge of metal in the molten state and
a furnace maximum power consumption, the method comprising the steps of:
supplying an AC electric power to said plurality of induction furnaces from
a plurality of inverter units equal in number to said plurality of
induction furnaces, wherein each inverter unit of said plurality of
inverter units dedicatedly supplies said AC electric power to a respective
furnace of said plurality of induction furnaces, and each inverter of said
plurality of inverter units further comprises a plurality of inverter
modules;
supplying a magnitude of DC electric power from at least one rectifier
module receiving a magnitude of AC electric supply power to all of said
plurality of inverter units, said magnitude of DC electric power equal to
less than the sum of the furnace maximum power consumption for each
furnace for all of said plurality of induction furnaces, and greater than
or equal to the sum of said furnace maximum holding power for each furnace
of all of said plurality of induction furnaces;
establishing a power set point for each furnace of said plurality of
induction furnaces;
reducing said power set point for each furnace of said plurality of
induction furnaces for which the power set point exceeds said furnace
maximum holding power if the summation of said power set point for each
furnace for all of said plurality of induction furnaces is greater than
said magnitude of DC electric power; and
sharing a magnitude of available melting power among each furnace of said
plurality of induction furnaces having a power set point greater than said
furnace maximum holding power wherein said magnitude of available melting
power is determined by the difference between said magnitude of AC
electric supply power and a total power required for holding determined by
summing the power set point for each furnace of said plurality of
induction furnaces wherein said power set point is less than or equal to
said furnace maximum holding power for each furnace of said plurality of
induction furnaces.
19. A method for selectively melting metal and holding molten metal in a
plurality of induction furnaces, each furnace of said plurality of
induction furnaces having a furnace maximum holding power to hold a
substantially full charge of metal in the molten state and a furnace
maximum power consumption to melt a substantially full charge of metal,
comprising the steps of:
supplying an AC electric power to said plurality of induction furnaces from
a plurality of inverter units equal in number to said plurality of
induction furnaces, wherein each inverter unit of said plurality of
inverter units dedicatedly supplies said AC electric power to a respective
furnace of said plurality of induction furnaces, and each inverter of said
plurality of inverter units further comprises a plurality of inverter
modules;
supplying a magnitude of DC electric power from at least one rectifier
module receiving a magnitude of AC electric supply power to all of said
plurality of inverter units, said magnitude of DC electric power equal to
less than the sum of the furnace maximum power consumption for each
furnace for all of said plurality of induction furnaces, and greater than
or equal to the sum of said furnace maximum holding power for each furnace
of all of said plurality of induction furnaces;
establishing a power set point, P.sub.i, for each furnace of said plurality
of induction furnaces;
reducing said power set point, P.sub.i, for each furnace of said plurality
of induction furnaces for which the power set point, P.sub.i, exceeds said
furnace maximum holding power if the summation of said power set point for
each furnace for all of said plurality of induction furnaces is greater
than said magnitude of DC electric power;
inputting a percentage of demand limitation, D%, and a total maximum power,
P.sub.total, available to the system;
determining a total power required for holding, .SIGMA.P.sub.hold, by
summing the power set point, P.sub.i, for each furnace of said plurality
of induction furnaces having said power set point, P.sub.i, less than or
equal to said furnace maximum holding power;
determining a total power required for melting, .SIGMA.P.sub.melt, by
summing the power set point, P.sub.i, for each furnace of said plurality
of induction furnaces having said power set point, P.sub.i, greater than
said furnace maximum holding;
determining a total power available for melting, P.sub.available, from the
equation:
P.sub.available =[D% .circle-solid.P.sub.total ]-.SIGMA.P.sub.hold ;
determining a power availability coefficient, k.sub.available, from the
equation:
k.sub.available =[P.sub.available ]/[.SIGMA.P.sub.melt ]; and
adjusting said power set point, P.sub.i, for each furnace of said plurality
of induction furnaces for which said power set point, P.sub.i, is greater
than said furnace maximum holding power to a value substantially equal to
the product of said power availability coefficient, k.sub.available, and
said power set point, P.sub.i, if said power availability coefficient,
k.sub.available, is less than unity.
Description
FIELD OF THE INVENTION
The invention relates to induction furnaces, and especially to an improved
power supply system for large systems using induction furnaces for the
melting and heating of metals.
BACKGROUND OF THE INVENTION
Induction melting systems have gained in popularity in the production of
metal cast parts, because they provide an efficient and clean way to heat
and melt metals. In an induction furnace, a metal charge is placed inside,
or immediately adjacent to, an induction coil. Electric power is supplied
to the induction coil, and heats the metal by means of the electromagnetic
field produced by the coil. Modern induction melting systems include an
induction furnace supplied with power through a solid state power
converter. The solid state power converter converts three-phase
standard-frequency (50 or 60 Hz) power, from a public power utility's
distribution line or the like, into single-phase variable-frequency
current applied to the coil. The converter adjusts the variable frequency
to match the inductive impedance of the coil with the capacitive impedance
of the power supply to deliver optimum power to the metal charge inside
the furnace.
Advances in power semiconductor devices have made it possible to build
larger static solid state converters capable of providing megawatt level
power. This facilitates production rates of tens to hundreds of tons of
molten metal per hour.
The increase in size ofinduction melting systems poses two major
requirements--continuous uninterrupted supply of molten metal "on-spec"
from the melt shop to the cast shop and high reliability of the melting
system. U.S. Pat. No. 5,272,719 describes a dual output converter that
allows a single transformer/rectifier unit to deliver power to two
furnaces simultaneously and allows the power to be shifted smoothly
between the two furnaces.
This concept may be further expanded to allow the use of a plurality of
inverter units, each supplying a separate furnace, with one rectifier
unit. A large melt shop may require production of 100 tons of molten metal
each hour consuming about 50 megawatts. To assure a constant and steady
supply of metal, the system may consist of a transformer/rectifier unit
capable of converting 50 megawatts from AC line to DC and three
24-megawatt inverters each connected to a 35 ton furnace. With such an
arrangement, full power melting can be conducted in two furnaces
simultaneously, consuming 48 megawatts, while 2 megawatts can be applied
to maintain the temperature in the third furnace which is in the process
of dispensing hot molten metal.
SUMMARY OF THE INVENTION
In accordance with the present invention, system reliability is achieved
via modular design of the transformer/rectifier units and/or of the
inverter units. Each unit may consist of several self-contained modules.
According to one aspect of the invention, there is provided a system for
melting metal and holding molten metal. The system comprises a rectifier
unit receiving AC electric power and outputting DC electric power; one or
more inverter units, all receiving the DC electric power output from the
rectifier unit and each outputting AC electric power; and one or more
induction furnaces each receiving the electric power output from a
respective one of the inverter units. Each inverter unit preferably
comprises a plurality of inverter modules connected in parallel, each said
module independently being connectable to and disconnectable from the
rectifier unit and the respective furnace. Instead, or in addition, the
said rectifier unit preferably comprises a plurality of rectifier modules
connected in parallel, each said module independently being connectable to
and disconnectable from the power supply and the rectifier units.
Each rectifier module may comprise a transformer and a rectifier to rectify
the output from the transformer. At least some of the transformers are
then preferably equipped to shift the phase of AC power while transforming
it; and the transformers in different said rectifier modules then
preferably shift the phase of the AC power supply by different amounts.
Each rectifier or inverter module may have a controller, with the
controllers communicating via a network, and when a problem affects one
module, the respective controller may then disconnect that module and
notify the controllers of other modules of the same unit over the network.
When the said other controllers are notified that one module has been
disconnected, they may shut down the remaining modules one after another.
Instead, when one module is disconnected the remaining modules may
continue to supply power at a reduced total capacity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a conventional induction melting system.
FIG. 2 is a block diagram of a previously proposed induction melting
system.
FIG. 3 is a block diagram of an induction melting system having a
tri-output power conversion system.
FIG. 4 is a block diagram of an induction melting system having modular
transformer/rectifier and inverter units;
FIG. 5 is a schematic perspective view of the system of FIG. 4.
FIG. 6 is a perspective view of a detail of FIG. 5.
FIG. 7 is a block diagram of a control network for the system of FIG. 4;
FIG. 8 is a schematic circuit diagram of a transformer/rectifier module of
the system of FIG. 4.
FIG. 9 is a schematic circuit diagram of inverter modules of the system of
FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 of the accompanying drawings, one conventional form of
an induction melting system comprises a power converter that consists of
two units: a transformer/rectifier 12 and an inverter 14. The
transformer/rectifier 12 converts AC line power from a 3-phase supply 16
into DC power on a line 18, and the inverter 14 converts the DC power into
AC current supplied to an induction furnace 20. To compensate for the
inductive impedance of the furnace 20, the inverter unit 14 incorporates
capacitors (not shown) which, when taken together with the furnace
inductance, form a resonance loop. Varying the operating frequency in the
inverter 14 alters the impedance of the resonance loop, thus controlling
the power delivered to the load 20.
As shown in FIG. 2, a dual output induction melting system similar to that
disclosed in U.S. Pat. No. 5,272,719 (the entire content ofwhich is herein
imported by reference) comprises a single transformer/rectifier 12. The
transformer/rectifier 12 supplies in parallel two inverters 14 through the
common DC lines 18. Each of the inverters 14 supplies a separate furnace
20. Each inverter 14 is capable of supplying the full melting power to its
furnace 20. The maximum power output of the transformer/rectifier 12 is
sufficient to supply the full melting current to one furnace, and a lower
power, for sintering or other processing or simply to maintain the
temperature of a charge of molten metal, to the other furnace. The two
furnaces are operated with an alternating cycle, so that one is melting
metal while the other is processing or dispensing metal previously melted.
As shown in FIG. 3, an improved induction melting system has a tri-output
transformer/rectifier 12 supplying three inverters 14, each ofwhich
supplies a single furnace 20. As an example, each of the furnaces 20 is a
35-ton furnace, requiring 24 MW for 40 minutes to melt a charge of metal,
and requiring 2 MW holding power to keep the molten charge hot. Thus, each
of the inverters 14 has a power throughput capacity of 24 MW. The
transformer/rectifier 12 has a power throughput capacity of 50 MW. By
suitably staggered timing of the operating cycles of the three furnaces,
one furnace can then start a melting cycle every 20 minutes. The shop may
then empty one furnace by pouring for 20 minutes, while the other two
furnaces are melting the next two batches. The cast shop then has a steady
supply of metal, with each furnace becoming available for pouring just as
the previous one is emptied. The electric power consumption is also
steady, minimizing the power demand.
However, the system shown in FIG. 3 presents problems. In particular, the
very large, 24 MW inverters are not entirely reliable and durable, and the
failure of one of the inverters can cause operating problems, if a furnace
containing 35 tons of molten or partly molten metal is abruptly deprived
of heat. Such a failure also deprives the system of a third of its
metal-melting capacity, and an abrupt failure may result in an
unacceptable transient surge propagating back into the incoming power
supply 16. A failure in one of the phases of the three-phase
transformer/rectifier 12 could have similar undesirable effects.
Referring now to FIGS. 4 to 6, in one embodiment of the present invention
the single transformer/rectifier unit 12 consists of eight modules 22
arranged in parallel. Each module 22 is connected to the incoming 3-phase
AC line 16, and to the DC line 18.
Each of the inverters 14 consists of eight modules 24 arranged in parallel.
Each module 24 is supplied by the DC line 18, which thus forms a DC power
bus connecting all eight transformer/rectifier modules 22 to all
twenty-four inverter modules 24. All eight inverter modules 24 making up
an inverter 14 supply, through another bus 25, the induction coil of their
respective furnace 20.
As an example, a 50 megawatt three-output melting system shown in FIG. 4
consists of one transformer/rectifier unit and three inverter units. The
transformer/rectifier unit 12 consists of eight identical
transformer/rectifier modules 22 each rated at 6,500 kVA. Each inverter
unit 14 consists of eight 3-megawatt modules 24.
The block diagram of the high power modular induction heating and melting
system is shown in FIG. 4. The physical layout of this system is shown in
FIGS. 5 and 6 and the control block diagram is shown in FIG. 7.
Each transformer/rectifier module 22 includes an autonomous electronic
controller 26 and each inverter module 24 includes an autonomous
electronic controller 28. All of the controllers communicate via a shared
digital control network shown in FIG. 6. In addition, an input line
monitor 30, furnace monitors 32, and operator interface panels 34 are
connected to the control network. The task of the monitors and controllers
is to assure safe operation of the entire system.
Referring now to FIG. 8, each transformer/rectifier module 22 comprises
high voltage disconnection switches 42, protection fuses 44, a transformer
46, a rectifier 48, a DC reactor 50, DC disconnection switches 53, and the
controller 26.
The high voltage disconnection switches 42 serve to safely disconnect the
transformer/rectifier module 22 from the incoming high voltage three-phase
line 16 and allow servicing of the module. The protection fuses 44 serve
to protect the transformer 46 from overload conditions. The transformer 46
serves to match the primary AC line voltage with the secondary voltage
needed by the rectifier 48 to produce a stable DC supply for the system.
The transformer 46 contains primary windings 52 and secondary windings 54
that can be configured to produce a desired phase shift between primary
and secondary AC voltages.
Each module 22 has a different phase shift on the secondary winding 54
produced by interconnection of different sub-windings 56. By shifting the
phases it is possible to minimize the distortions that the rectifier 48
injects into the power line 16. This reduction of distortions is achieved
when distortions produced in one of the rectifier modules 22 are negated
by distortions produced in another rectifier module 22. Therefore, the
distortions are trapped inside the secondary windings and circulate among
the transformer rectifier modules without reaching the high voltage supply
line.
The rectifier 48 comprises a set of six silicon control rectifiers (SCRs)
58 which are activated from the rectifier controller 26 via an SCR
integrated gate terminal 60. The controller 26 monitors the AC, DC, and
SCR voltages as well as AC current in the secondary windings of the
transformer. The controller 26 includes a digital communications interface
(not shown) that allows the controller to communicate with other
controllers and monitors in the system as wells as with external operator
panels and supervisory computers.
The rectifier controller 26 can activate the rectifier SCRs 58 upon
external command or can shut them off when an abnormal situation endangers
safe operation. Only the rectifier module 22 that is under threat will be
shut down, while the rest of the modules 22 can continue to operate. The
system as a whole can then continue to work, although possibly at reduced
output.
The controller 26 in the affected module 22 will inform other controllers
of the removal from service of one transformer/rectifier module, and
appropriate adjustments in the entire system can take place either
automatically or at the direction of a human operator at a panel 34.
The DC reactor 50 suppresses DC ripple induced by the inverter on the DC
bus 18 to minimize the injection of the inverter frequency into the AC
supply line 16. The DC reactor 50 also limits the rate of change of the DC
current during startup of the melting system and if the DC voltage
collapses.
The controllers 26 also adjust the timing of the SCR gating to equalize
power consumption among all of the transformer/rectifier modules 22. The
high voltage and DC disconnection switches 42 and 53 are manually
operated. They are used when one of the modules 22 has to be removed from
service and disconnected from the power supply for servicing.
Referring now to FIG. 9, each of the inverter modules 24 includes a panel
consisting of two inverter grade SCRs 62, a set of diodes 64, a set of
commutation reactors 66, a set of primary-capacitors 68, a set of
secondary capacitors 70, a set of filter capacitors 72, an electronic
disconnection switch 74, mechanical disconnection DC switches 76, and
mechanical disconnection output switches 78. The electronic disconnection
switch is implemented by means of a high voltage diode, SCR, Gate Control
Thyristor (GCT) or MOS Controlled Thyristor (MCT).
The inverter SCRs 62 are fired alternatively, injecting AC current into the
AC bus that leads to the induction furnace 20. The furnace current is
almost equally distributed among the inverter modules 24. The deviation in
the furnace current is determined by the tolerances in the primary and
secondary capacitance in each inverter module:
##EQU1##
where: I.sub.i is the AC current in each inverter module;
I=.SIGMA.I.sub.i is the total AC current of all inverter modules in the
induction furnace;
.DELTA.C.sub.i is the relative tolerance of the capacitors in each module;
N is the number of inverter modules in the inverter unit.
Typical tolerances of AC capacitors are in the range of 5% of average
value. Therefore, inverter current in each module is in the range of
I.sub.i =I/N(1.+-.0.05).
The gate pulses in all inverter modules are synchronized. The inverter
controllers 28 monitor the DC and AC voltages and the currents in inverter
module components. The inverter controllers 28 are connected via the
network with each other and with the operator panels 34. If and when an
inverter controller 28 detects abnormal conditions in an inverter module
24, it may first disconnect this module from the DC bus by opening the
electronic disconnection switch 74. In the event of failure in one of the
inverter SCRs 62 and a short in the inverter panel 24, the electronic
switches automatically block discharge of energy from one inverter module
to another. The fault is also reported via the control network to all
other modules comprising the same inverter unit 14. Depending on the
particular circumstances and the control algorithms in use, the other
modules 24 may then continue to supply the furnace 20 with somewhat
reduced melting power, lower the power to a holding level at which melting
is suspended but the molten material within the furnace is prevented from
solidifying, or stop their operation entirely. The reason for the fault is
reported on at least the nearest operator interface terminal 34. Even if
the decision is to shut down the furnace immediately, that is actually
done by stopping the operation of the inverter modules 24 one at a time at
slight intervals, so that the switching transients injected back into the
AC supply line 16 or inflicted on other parts of the system are greatly
reduced.
The manual AC and DC disconnect switches 76 and 78 allow an individual
inverter module 24 to be removed from service for servicing or repair.
The control algorithm described below is based on factory preset parameters
and process variables. The principle of the algorithm is to supply
sufficient power to all furnaces to hold metal from freezing. The
remainder of the available power is distributed among those furnaces that
are engaged in melting proportionally to the amount of power that each
such furnace requests. When the total power that may be drawn from an
external utility supply is limited, the total maximum power available to
the induction heating and melting system may have to be reduced below its
nominal value. That may occur if, for example, other loads sharing the
utility supply are unusually high. That is effected by imposing a demand
limitation on the total maximum power available. Even in that case,
however, the furnaces holding molten metal are still to be supplied with
holding power.
Factory Set Parameters
N--Number of furnaces or inverter units
P.sub.total --Total maximum power available to the system
P.sub.max --Maximum power rating of each inverter
P.sub.hold --Maximum holding power for each furnace
Control Variables
P.sub.i --Power set point for each furnace
n.sub.hold --Number of units on holding power
n.sub.melt --Number of units melting
.SIGMA.P.sub.hold --Total power required for holding metal
.SIGMA.P.sub.melt --Total power for melting
D%--Demand (percent)
P.sub.available --Total power available for melting
k.sub.available --Power availability
The software monitors P.sub.i --power setpoints of all units on line. If
P.sub.i <P.sub.hold the value P.sub.i is added to the total holding power
P.sub.hold and the count n.sub.hold of number of holding units is
incremented. Otherwise, the total power requested for melting P.sub.melt
is accumulated and the count n.sub.melt of number of melting units is
incremented. This way, each controller will know how many units are
melting and how much power they need.
The available power for melting:
P.sub.available =D%.multidot.P.sub.total -.SIGMA.P.sub.hold
and melting availability coefficient are computed
##EQU2##
If k.sub.available <1, the power limit is set as a portion of requested
power
P.sub.i limit =k.sub.available .multidot.P.sub.i
Pseudocode:
for i=1 to N
{if P.sub.i <P.sub.hold {(P.sub.hold +P.sub.i ;}
else {.SIGMA.P.sub.melt +P.sub.i ;}
k.sub.available (D%*P.sub.total -.SIGMA.P.sub.hold)/.SIGMA.P.sub.melt
If (P.sub.i *k.sub.available >P.sub.hold and k.sub.available <1) {P.sub.i
limit =k.sub.available *P.sub.i }
else {P.sub.i limit =P.sub.hold }
Demand Control is fed into all control units. If one of the rectifier
modules is removed for service, the power demand is reduced to (m-1)/m of
its normal value where m=number of rectifier modules in the system.
Reference is made to the article "Tomorrow's Induction Melt Technologies
Today" by John H. Mortimer, P.E. in Foundry Management and Technology,
March 1999 pages 14-20 and May 1999 pages 41-50. The entire content of the
article is herein incorporated by reference.
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